Pulp is washed for eliminating the black liquor in the pulp as effectively as possible. The sub-process of washing thus decreases consumption of chlorine dioxide which is used in the bleaching sub-process. The washing loss a.k.a carry-over, which refers to the amount of organic matter in the pulp and which causes a demand of chlorine dioxide in bleaching, has been measured as COD (Chemical Oxygen Demand) load on the basis of electrical conductivity of unbleached pulp. Another method for determining COD load is based on a measurement of an optical property of unbleached pulp. However, the present methods are old fashioned and not accurate enough for environmentally friendly bleaching required today.
Thus, there is need to have a better solution to measurement of chemical demand for chlorine dioxide.